April 2008

Mendelian GeneticsMendelian Genetics

March 11, 2009March 11, 2009Mr. BromwellMr. Bromwell

Drill – March 11, 2009Drill – March 11, 2009Which of the following genotypes are homozygous? heterozygous?

Aa BB cc Dd ee Ff GG

In peas, purple flower color is dominant to white flower color.

• What would be the genotype of a plant with white flowers?• What would be the phenotype of a plant with the genotype ‘Pp’?

Which one of Mendel’s Laws explains why some traits in pea plants appear to ‘skip’ generations? Explain the law.

Mendel’s ExperimentMendel’s Experimentx

Using Mendel’s Laws to PredictUsing Mendel’s Laws to Predict• Geneticists can use Mendel’s Laws and the rules of

probabilityprobability to predict whether an allele (or alleles) will be passed from parent to offspring.

probability (n.) – the likelihood that an event will occurprobability (n.) – the likelihood that an event will occur

• Probability can be expressed as…• a fraction (1/2),• a percentage (50%),• or a ratio (1:1).

Making Predictions - DimplesMaking Predictions - DimplesProblem:Having dimples (depressions in the cheek when you smile) is a dominant trait controlled by a single gene.

Two parents have dimples and are heterozygous for the trait. What is the probability that they will have a child with dimples?

Making Predictions - DimplesMaking Predictions - DimplesSolution:From the information in the problem you know that both parents have the heterozygous genotype ‘Dd’.

D = dominant allele (dimples)d = recessive allele (no dimples)

From here there are two ways to solve the problem – using basic probability or using a Punnett square.The Punnett squarePunnett square, developed by Sir Reginald Punnett in the early 1900s, is pictured at right.

D d

D DD Dd

d Dd dd

♂ male gamete types

♀ fe

mal

e ga

met

e t

ypes

A Word About RatiosA Word About RatiosA cross that involves two hybrids (heterozygous parents) for a single trait is called a monohybrid crossmonohybrid cross.

There are always three possible genotypes in a ratio of 1:2:11:2:1.

There are two possible phenotypes in a ratio of 3:13:1.

D d

D DD Dd

d Dd dd

♂ male gamete types

♀ fe

mal

e ga

met

e t

ypes

Pea Plants Provide Practice ProblemsPea Plants Provide Practice Problems1. In peas, round-shaped peas are dominant to wrinkled peas. A

farmer takes a pea plant that is heterozygous for round peas and breeds it with a pea plant that produces wrinkled peas. What is the probability that the offspring will produce round-shaped peas?

2. Tall pea plants are dominant to short pea plants. Two heterozygous tall pea plants are bred together. What is the probability that the offspring will be heterozygous like their parents?

3. Green pea pods are dominant to yellow pea pods. A farmer has two plants with green pods. He crosses them in the hopes of getting some plants with yellow pods? Is this possible? Explain our answer.

ReferencesReferencesBiggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.Szeto, C. “Asian (Oriental) American girl with dimples” Wikimedia Commons. Jan 21, 2008. Retrieved Apr 6,

2008 from <http://commons.wikimedia.org/wiki/Image:Asian_girl_with_dimples.jpg>.